Stress under normal conditions, hypokinesia simulating weightlessness, and during flights in space

Stress due to intensive mental work under normal conditions was compared to stress under a sharp limitation of motor activity (hypokinesia), simulating weightlessness on the human body. Mental stress causes typical alterations of cerebral circulation under normal conditions: increase of blood flow in the supramarginal and angular gyri of the parietal lobe, in the frontal lobe, and in the superior temporal gyrus of the left hemisphere, and changes in cardiac activity and in the tonus of vessels. Dynamics of human stress reactions, among other features of this process, is best reflected in the parameters of a electrocardiogram, a rheoencephalogram, and total peripheric vascular resistance. An increase in the latter is an informative index of stress development. Human reaction to stress under hypokinesia and during flights in space have specific features. Prolonged hypokinesia causes an imbalance in an organism's control systems, specifically depressor reactions are distorted. In the context of hypokinesia, anxiety and mental stress lose their adaptive nature to a large extent. They provoke disturbances of the heartbeat and hypertensive reactions. A whole complex of factors affects the living organism during space flights. An imbalance of the body's control systems, emotional and physical overloads, which arise episodically, changes in electrolyte and energetic metabolism, and alterations in the head vessels increase the probability of reactions to stress and reinforce their effect. Stress can be retarded by using on elaborated system of preventive measures which includes physical training, psychological support of astronauts and, to some degree, reduction of the hypothalamus adrenergic centers' tonus through muscle relaxation. Astronauts' reactions to being in space occur during flights under heavy loading tests and in emergency situations. Weightlessness does not generate stress when one has adapted to it. Returning from weightlessness to the Earth's gravitation causes stress. After prolonged flights, stress associated with readaptation to the Earth's gravitation is atypical in character (increase of sympatoadrenalic system activity against the background of a reduction in hypothalamo-hypophysial system activity). We explain the voltage decrease of the T-wave of the electrocardiogram, the phenomenon repeatedly occurring both during prolonged space flights and under hypokinesia, by a lowering of cardiomyocytes, energetic potential due to hypokalemia, insufficient glucose usage, and a decrease in the coupling of oxidative phosphorylation processes. [Translated from Fiziologiya Cheloveka, vol. 22, no. 2, 1996, p. 10-19]

Effects of 28-day head-down tilt with and without countermeasures on lower body negative pressure responses

This study was performed to determine the effects of 28 d of head-down tilt (HDT) (simulated weightlessness) on cardiovascular responses to orthostatic stress induced by lower body negative pressure (LBNP) (before, during, and after HDT) and +60 degrees head-up tilt (before and after HDT) in 12 subjects. Half of them underwent countermeasures (CM) of regular muscular exercise (isometric and isokinetic training) and LBNP sessions (-30 mm Hg) as generally performed during spaceflight; the other six were a control group (C). The countermeasure effect on the orthostatic responses to LBNP and tilt test was assessed by studying the changes after HDT in the two groups. Essentially, blood pressure was better maintained in group CM in the tilt test after HDT (MBP at the end of the tilt vs. baseline value: +16% (CM); -19% (C)). LBNP and muscular exercise may have contributed to this improvement. One of the probable contributing factors is the relative conservation of plasma volume, at the end of HDT, in group CM (-2.2%), compared to group C (-11.2%). Transcranial Doppler (TCD) recordings of middle cerebral artery (MCA) velocities permitted indirect evaluation of cerebral blood flow changes during the orthostatic tests. MCA velocities decreased significantly although slightly (-7 to -12%) during LBNP sessions without changes along the HDT showing that the cerebral circulation was well preserved in each group. On the other hand, subjects undergoing presyncopal symptoms presented a drop in MCA velocities, suggesting a decrease in cerebral blood flow.

Electrical vestibular stimulation and space motion sickness

Electrical vestibular stimulation (EVS) in dynamic balance condition was studied in order to search for a new provocative test of space motion sickness (SMS). SMS is usually attributed to a sensory conflict caused by exposure to microgravity. Vestibular information is conflicting but also unusual and insignificant. EVS is in accordance with this feature because it is not the adequate stimulus of the vestibular receptors. EVS was achieved by means of binaural electrical stimulation. Effects of EVS were potentiated by compelling the subject to maintain dynamic balance on a seesaw. The quantification of this function was performed before during and after EVS in order to investigate a possible relationship between objective consequences of EVS i.e. dynamic balance disturbances, and the discomfort experienced by the subjects. Dynamic balancing skill was statistically worsened during EVS. Moreover EVS evoked subjective symptoms of SMS in 17 out of the 30 subjects examined. During EVS in eyes open conditions, the subjects who encountered the strongest discomfort, presented the most disturbed dynamic balance, evidencing a relationship between the level of discomfort and the imbalance arising from EVS This method could thus constitute an interesting basis of SMS ground-based test.

Volume loading of the heart by "leg up" position and head down tilting (-6 degrees) (HDT)

Head down tilting is widely used to increase preload and to induce intrathoracic blood pooling similar to microgravity. During daily routine, this venous pooling is performed by raising the legs up. In this study, both these approaches have been compared by invasive measurement using a right heart catheter. In patients with moderate coronary artery disease, diagnostic right heart catheterization was performed by the Swan-Genz-techniques. All measurements were performed with head down tilting (-6 degrees) and with "leg up" position. Patients then received Nitroglycerin to countermeasure the preload changes. Pressures in the pulmonary artery as well as in the wedge position increased significantly during leg up and HDT. However, changes were significantly more pronounced in the "leg up" position than during HDT. No changes were observed for arterial blood pressure, cardiac output, stroke volume and resistances. Nitroglycerin during HDT lowered blood pressure and pressures in the pulmonary artery and in PCW-position and reduced cardiac output significantly. Both approaches of volume loading of the heart induced significant changes and increases of preload. However, changes were more pronounced during the "leg up" position than during HDT. It is questioned whether HDT with -6 degrees is appropriate to truly reflect hemodynamic alterations during simulated weightlessness.

Hemodynamic effects of microgravity and their ground-based simulations

Hemodynamic effects of simulated microgravity were investigated, in various experiments, using radioactive isotopes, in which 40 healthy men, aged 35 to 42 years, took part. Blood shifts were evaluated qualitatively and quantitatively. Simulation studies included bedrest, head-down tilt (-5 degrees and -15 degrees), and vertical water immersion, it was found that none of the methods could entirely simulate hemodynamic effects of microgravity. Subjective sensations varied in a wide range. They cannot be used to identify reliably the effects of real and simulated microgravity. Renal fluid excretion in real and simulated microgravity was different in terms of volume and time. The experiments yielded data about the general pattern of circulation with blood displaced to the upper body.

The involvement of cerebrovascular reactivity in pathogenesis of space motion sickness

The aim of the presented work is to investigate the mechanisms of involvement of cerebral circulation in pathogenesis of space motion sickness. In experiments on chronic rabbits the 2-hours rocking and antiorthostasis served as a model of influences of the zero-gravity conditions intracranial hemo- and liquorodynamics. Cerebravascular reactivity was evaluated as a response of cerebral blood flow and intracerebral rheoencephalography to functional loads (photostimulation, CO2-inhalation, Stookey test). Shifts of water balance in the brain tissue has been assessed by means of three-frequency brain electrical impedance. Rocking plus aniorthostatis resulted in decrease of the cerebrovascular reactivity and hyperhydration. The data obtained show a significant decrease of compensatory abilities of the cerebrovascular system under the zero-gravity conditions.

Carotid-cardiac baroreflex: relation with orthostatic hypotension following simulated microgravity and implications for development of countermeasures

In a series of studies, we have examined the effects of exposure to simulated microgravity, varying states of vascular volume, and acute exercise on the function of the carotid-cardiac baroreflex in man. In the first study, exposure to simulated microgravity (6 degrees headdown bedrest) reduced the sensitivity and buffer capacity of the vagal baroreceptor-cardiac reflex mechanisms and this impaired baroreflex function was associated with orthostatic hypotension. Since the reduction in plasma volume during BR was not correlated with impaired baroreflex function, a second study was conducted which demonstrated that the carotid-cardiac baroreflex response was not affected by either acute hypovolemia or hypervolemia. These results suggest that acute fluid replacement prior to reentry may not reverse impaired baroreflex function associated with postflight hypotension. In a third study, we demonstrated that one bout of maximal exercise increased baroreflex sensitivity and buffer capacity through 24 h post-exercise. These baroreflex changes were opposite to those observed following BR. Taken together, these data suggest that the contributions of reduced blood volume and impaired carotid-cardiac baroreflex function to orthostatic hypotension following exposure to microgravity are probably separate and additive; maximal exercise in addition to fluid replacement may provide an acute effective countermeasure against postflight hypotension.

Does vector-free gravity simulate microgravity? Functional and morphologic attributes of clinorotated nerve and muscle grown in cell culture

Cocultured Xenopus neurons and myocytes were subjected to non-vectorial gravity by clinostat rotation to determine if microgravity, during space flights, may affect cell development and communications. Clinorotated cells showed changes consistent with the hypothesis that cell differentiation, in microgravity, is altered by interference with cytoskeleton-related mechanisms. We found: increases in the myocyte and its nuclear area, "fragmentation" of nucleoli, appearance of neuritic "aneurysms", decreased growth in the presence of "trophic" factors, and decreased yolk utilization. The effects were most notable at 1-10 rpm and depended on the onset and duration of rotation. Some parameters returned to near control values within 48 hrs after cessation of rotation. Cells from cultures rotated at higher speeds (>50 rpm) appeared comparable to controls. Compensation by centrifugal forces may account for this finding. Our data are consistent, in principle, with effects on other, flighted cells and suggest that "vector-free" gravity may simulate certain aspects of microgravity. The distribution of acetylcholine receptor aggregates, on myocytes, was also altered. This indicates that brain development, in microgravity, may also be affected.

The action of simulated and true weightlessness on the digestive tract of rats

Rats on board the Soviet Cosmos 936 satellite for l8 1/2 days showed a decreased glycoprotein secretion from the salivary mucous glands, stomach and intestine, and an increased leucine aminopeptidase and acid phosphatase content from the small intestine. Grimelius positive cells were activated. One group of rats were centrifuged at 1 g during the flight to simulate terrestrial gravity. Some investigations have suggested that under these conditions muscular and cardiac disorders diminished. In the digestive tract the benefits of centrifugation at 1 g are minimal and limited to a few glycoprotein components. The digestive changes are probably the expression of a stress response, unrelated to weightlessness. Similar changes, concomitant with a glycocorticoid hypersecretion, were found in rats after 15 days of hypokinesia on Earth. These digestive changes persisted even in adrenalectomized rats.

Cardiorespiratory responses to exercise after bed rest in men and women

The purpose of this study was to compare cardiorespiratory responses of men and women to submaximal and maximal workloads before and after bed rest (BR). Fifteen male college students (19-23 yr) and 8 female nurses (23-34 yr) underwent 14 d and 17 d, respectively of bed rest. The maximal work capacity test was performed in the supine position on a bicycle ergometer just before and immediately after bed rest. The women's maximal O2 uptake (maximal VO2) was 41% lower (P<0.05) than the men's before bed rest and 42% lower (P<0.05) after bed rest. During bed rest the women's maximal VO2 decreased from 2.06 to 1.86 liter/min (-9.7%, P<0.05), and that of the men decreased from 3.52 to 3.20 liter/min (-9.1%, P<0.05). Compared with pre-BR values, after bed rest the maximal ventilatory volume was essentially unchanged in the men (+1.8%) and women (+ 2.3%), but maximal heart rate was elevated from 185 to 193 b/min (+ 4.3%, P<0.05) in the men and from 181 to 187 b/min (3.3%, P<0.05) in the women. Submaximal VO2 was unchanged after bed rest in the men but was significantly reduced in the women; the women's Hct and RBC levels were lower (P<0.05) than comparable male data. Mean corpuscular volume was unchanged in both groups pre- and post-bed rest. It is concluded that the proportional deterioration in maximal VO2 following prolonged bed rest was essentially the same in young men and women.